Scouting and management

Because grasshopper populations often build in non-crop areas and later move into soybean, field edges are usually the first areas to show feeding injury, and where sampling efforts can initially be focused.

Begin scouting for grasshoppers after plants emerge. Concentrate sampling on the field edge and count the number of adults and nymphs in a one square foot area. Repeat this for a total of 20 samples. To determine the number of grasshoppers/square yard (which thresholds are defined as), multiply your average number of grasshoppers/square foot by nine.

As plants get larger, visually inspect plants for defoliation. To estimate defoliation, examine a minimum of 10 plants.

To estimate percent defoliation:

From each each plant, select a leaf from the top, middle and bottom thirds of the plant.

Use Figure 3 to estimate percent defoliation for each leaf and determine the average percent defoliation across the three leaves from each plant.

This average percent defoliation for the field's canopy can be compared to treatment thresholds.

Risk for infestation by grasshoppers is greater in years following long, warm autumns and warm, dry springs. Populations tend to build over multiple years, so high populations observed in one year could indicate higher risk the next year.

Figure 3. Percent defoliation of soybean leaves. Photo: Robert Koch, University of Minnesota

Thresholds

For defoliation-based thresholds, the following recommendations apply: